A B767-200 Flight Attendant reported the R1 door would not close following catering; so the IRO assisted at which time the emergency slide inflated.

Narrative

All pilots were on the flight deck preparing for the return flight. The Lead Flight Attendant notified the crew that the R1 door would not arm. As IRO; and positioned closest to the flight deck door; I exited to inspect the door. I found the door to be closed and in the disarmed mode. I asked the Lead Flight Attendant to verify my observation prior to any closer inspection. With her concurrence that the door was disarmed I proceeded to visually inspect the door perimeter for obstruction. When none were found I moved my inspection to the door handle to make sure it was closed completely. While inspecting the door locking handle for full unobstructed closure; and despite the fact that the door had been dually verified to be in the disarmed mode; the R1 door escape slide deployed. Please note this event occurred prior to flight. I believe the foreign catering crew had mishandled the door in some manner. This door had no problem closing on the flight prior to this event or the flight after. They were the only ones to operate the door at this station. I would suggest that Maintenance address any non-normal door closures; openings; or door arming anomalies.